Living Small

You think your life is tough? Try swimming through roofing tar with a flimsy flagellum. David Dusenbery ’64 explores the everyday challenges of being a microbe.

BY william abernathy ’88

Life as a microbe, it turns out, is no picnic.

Bacteria and other tiny life forms live in the same world that we do, but at their size, the laws of physics operate quite differently. If you were shrunk down to the size of a microbe, water would have the apparent viscosity of lukewarm roofing tar, the resistance of moving through it vastly exceeding your puny inertia. If you were lucky enough to have an appendage, and could figure out how to wiggle yourself forward, you’d stop moving the instant you were done thrashing, grinding to a halt in about the width of an atom. The tremendous effort of locomotion might not be worth it, though, since you wouldn’t be able to steer anyway: your surroundings would be a mosh pit of molecules battering you randomly from all directions. The smallest microbes literally can’t move on their own, can’t orient themselves to steer, and can only get around by random diffusion. And yet, these are arguably the most successful life forms on the planet.

At micro scale, forces that most scientists consider afterthoughts become vitally important. At our size, for example, Brownian motion (the tendency of microscopic particles to jiggle around in water) and the Reynolds number (the ratio of an object’s inertia to the viscosity of the surrounding fluid) are little more than curiosities—topics learned for an exam, then relegated to the Engineering department. (While the Reynolds number sees some use in airfoil and pump designs, the only practical application of Brownian motion is powering the Infinite Improbability Drive in The Hitchhiker’s Guide to the Galaxy.) To a microbe, though, these physics footnotes are far more important than the gravity and inertia that rule life at our scale.

David Dusenbery ’64 has spent his scholarly career examining the bizarre world of the microbe from a physicist’s perspective. Now an emeritus professor of biophysics at Georgia Tech, Dusenbery has shown his squarely interdisciplinary approach to the ecology of very small organisms in Living at Micro Scale: The Unexpected Physics of Living Small.

The unorthodoxy of Dusenbery’s approach is to start not from direct observation of microbes, but rather from the strange physical laws that govern them. This line of inquiry yields surprising insights. At the smallest sizes, for example, his calculations show advantages for organisms shaped like spheres; at slightly larger sizes, for elongated ellipsoids or rods. He also demonstrates distinct disadvantages to flattened, disk-like ellipsoids. Observed species bear out his findings: the smallest organisms are, indeed, spherical, while platter-shaped bacteria are unknown to science.

Likewise, by applying information theory, he identifies a certain threshold of smallness below which it makes no practical sense to use pheromones to find a mate or to detect prey. Below this threshold, the chance of bumping into a pheromone molecule is too small, the cost of secreting one too dear, the biological difficulty of interpreting and acting on the information too great, and the possibility of it giving away one’s location to a predator—armed with such microbial super-weapons as chemical detection, locomotion, and steering—too terrible. It makes more sense for tiny microbes to surf the random tides of Brownian motion, reproduce asexually when they can, and hope not to attract too much attention. Again, these theories are borne out by the observed behaviors of small and very small organisms.

Dusenbery hiking Mt. Baker in 2009. “Hiking remains a big part of my life. I learned it from my parents who learned it at Reed in the 1930’s. At Reed, Outings Club was my main social activity.” His parents are Harris Dusenbery ’36 and the late Evelyn Dusenbery ’37; his sister, Diane Dusenbery Waggoner ’68, is also a Reedie.

Dusenbery suggests that the physics of the very small is crucial to understanding the everyday life of microbes—how they move around, how they find food, how they avoid suffocating in their own wastes. Indeed, he argues that microbes evolve physiologically, morphologically, and behaviorally to ecological boundaries described by laws of physics.

Now retired from a career that took him from Reed to the University of Chicago, Cal Tech, and ultimately to Georgia Tech, Dusenbery might not have seemed the most likely candidate for a career in biophysics. His senior thesis, which he wrote with Professor Dennis Hoffman [physics, 1959–90], was entitled Toward a New Type of Thin-Film Electron Emitter.

How does a solid-state physicist make the jump to biophysics? “At that time,” Dusenbery recounts, “the exciting physics seemed to be in the subatomic particles, which was done at expensive, one-of-a-kind machines involving hundreds of people. I was more interested in small-scale research where I had more control. Since I had always been interested in what animals know about their environment, biophysics seemed attractive.”

In addition to the firm footing in mathematics and physics he received at Reed, he credits Reed economics classes for a start down his eventual path. “Many of the same ideas,” he notes, “are used in ecology and evolutionary biology.” A Reed connection from professor (and later acting president) Byron Youtz [physics, 1956–68] to the University of Chicago’s nascent biophysics program set him on his way.

His interdisciplinarianism has not always been appreciated. “Physicists get it,” Dusenbery says of his method. “They are used to the idea of making approximations to simplify the problem enough to make analysis possible. This leads to improved understanding, even if the analysis is of something not exactly like the subject in nature. Biologists often don’t see the value of this kind of approach and complain that some of the assumptions don’t fit details they know about.”

This imposing specimen is a cyclops, a type of freshwater copepod belonging to the crustacean family. Copepods are among the tiniest organisms to be equipped with pheremone attraction. Smaller creatures generally have not evolved the high sensitivity necessary for pheremones to be an effective strategy for tracking down mates. “One of my childhood memories is breaking ice on the Reed pond to get some pond water to view in my first microscope, probably a Christmas present... Cyclops were the most impressive organisms I remember from the Reed sample.”

Biologists, Dusenbery notes, quoting oceanographer Peter Jumars, “are trained from Biology 101 onward to look for small differences among organisms. Physicists, on the other hand, are likely to want to know first the factors that explain the 90% similarity before they pursue the 10% dissimilarity.”

“Many biologists tend to think of organisms as magical things that can do anything, and the biologist’s job is simply to discover what things they, in fact, do.” Dusenbery says. “I think it is just as important to understand what they cannot do.”

Additionally, Dusenbery’s approach offers insight into larger animals and ecosystems, providing intriguing glimpses into problems such as why there are two sexes, how many predators an ecosystem can sustain, and how big apex predators can get. Indeed, he sees implications for engineers as well as biologists. “Engineers interested in designing micro-robots,” he observes, “will need to deal with many of the same issues that bacteria have overcome.”

Looking forward, Dusenbery hopes other scientists will appreciate the power of this kind of analysis. “This book develops several theories that make quantitative predictions of the kinds of organisms that should be successful,” he says. “What I’d really like is for researchers to do the hard work necessary to ... test the theories more rigorously. Hopefully, a new generation of students will be inspired to make these kinds of studies.”

It’s clear that future researchers, like the low-Reynolds-number creatures they’re studying, won’t get far by coasting.